1. Field of the Invention
The present invention generally relates to the liquid crystal display, and more specifically to a device and method for driving the liquid crystal display.
2. The Prior Arts
A liquid crystal display (LCD), due to its small form factor, low power consumption, and low heat dissipation, has been widely utilized on various electronic devices. Especially, as the LCD technology has advanced to surpass the limitations and disadvantages of conventional or other existing display technologies such as cathode ray tube (CRT) and light emitting diode (LED), the LCD has been considered to have great importance and potential for the future development of computers, mobile handsets, and other consumer electronic devices.
Generally, an LCD is made by two glass substrates with specially processed surfaces and liquid crystal molecules interposed therebetween. When applying different voltages on the electrodes of the glass substrates, the orientation, and therefore the transparency, of the liquid crystal molecules would vary accordingly. Because the liquid crystal molecules do not illuminate by themselves, a kind of backlight has to be employed. As the light radiated from the backlight passes through the liquid crystal molecules with different transparency, an image is thereby displayed.
More specifically, the structure and function of a thin-film transistor (TFT) LCD is described as follows. In general, a TFT LCD is a layer of liquid crystal interposed between two glass substrates. Color filters are installed on one of the glass substrates and transistors are built into the other glass substrate. The transistors function as switches and control the voltages applied on the liquid crystal molecules. When the transistors are turned on and voltages are applied, the liquid crystal molecules will have corresponding orientations and transparencies. Each pixel of the LCD display therefore has a specific brightness. The color filters attached to the glass substrate give each pixel the three colors red, green, and blue. These pixels exhibiting the colors red, green, and blue constitute the image displayed on the LCD.
As mentioned earlier, the LCD technology has advantages that are not available from the conventional CRT and existing LED display technologies. The LCD display, however, does have its own limitations. As mentioned earlier, under the influence of the electric fields established by the voltages applied on the electrodes of the glass substrates, the liquid crystal molecules develop corresponding orientations and therefore a texture is formed. Then, by the lights radiate from the backlight module installed behind the glass substrate, the pixels of the LCD display manifest various degrees of brightness and an image is thereby displayed. During this process, the applied voltages can reach their target values instantaneously. The liquid crystal molecules, however, require a period of time to develop the targeted orientations. The change of brightness of pixels therefore lags behind the change of voltages, causing a so-called delay phenomenon. As shown in FIG. 1, the applied voltage reaches its targeted value (referred to as targeted code in FIG. 1) almost instantaneously but the brightness of the pixel follows the smooth dotted curve. This delay phenomenon seriously affects the display quality of fast changing, dynamic images on a LCD display.
Conventionally, to overcome such delay phenomenon, an overdrive method is applied whose device structure is shown in FIG. 2. The device contains series-connected transistor and capacitors to form a controller in controlling the voltage level applied on the liquid crystal molecule. Then a higher voltage is applied so that the liquid crystal molecule can reach its targeted optical response faster. The LCD therefore has a faster response time so that the requirement for displaying fast changing, dynamic images can be fulfilled.
To further explain the overdrive method, please refer to FIG. 3. FIG. 3(a) is a characteristic graph showing the optical response of a LCD pixel (near the intersection of the gate line G1 and the data line D1 as shown in FIG. 2) driven by the overdrive device according to a prior art. The unit of the voltage in the following description is referred to as code. A code could be a μV (10−6 V) or other similar voltage unit. Assuming that, to make the LCD pixel to reach its targeted brightness, the targeted driving voltage applied to the LCD pixel is code 128, the optical response of the LCD pixel is depicted as the dotted curve (a). To accelerate the optical response speed of the LCD pixel, conventional overdrive methods use a “coaxing” approach. FIG. 3(b) is a waveform diagram showing the pulse waveform of the control voltage asserted by the overdrive device according to a prior art on the gate line G1 (shown in FIG. 2). FIG. 3(c) is a waveform diagram showing the pulse waveform of the driving voltage asserted by the overdrive device according to a prior art on the data line D1 (shown in FIG. 2). At the pulses of the control voltage, the corresponding driving voltage is applied on the LCD pixel. As shown in FIGS. 3(b) and 3(c), a higher driving voltage code 200 is applied first so that the optical response of the pixel follows an acuter dotted curve (b) to reach the targeted brightness faster than the dotted curve (a). Then the driving voltage is adjusted to code 128 so that the pixel maintains its targeted brightness. Please note that, for the foregoing overdrive method according to a prior art, the period of the control voltage is the same as the frame time. For example, if the frame rate of the LCD display is 60 Hz, the frame time and the control voltage period are both 16.7 ms. In other words, the application of the next control voltage pulse and therefore the next driving voltage can only be applied in the next frame time. The optical response time of the LCD pixel therefore cannot be shortened to be within a single frame time. This is the major limitation of the overdrive method according to a prior art.
Accordingly, the present invention is aimed at overcoming the limitations and disadvantages of the LCD overdriving methods according to prior arts.